quoting constants in php: "this is a MY_CONSTANT"

Sorry, that's not the way constants in PHP work. You can put variables in double quotes and heredocs but not constants.

I recomend you to use concatenation because:

1. When you use a variable into a double quotes string your visibility is not good;
2. When you use a double quotes string the php can to process slowly;
3. You don't use a constant into a string, because don't have any delimiter to the php knows what is the constant.